Firing shrinkage of ceramic ware is a function of temperature history; therefore, local differences in the temperature of the ware during the firing cycle can produce non-uniform shrinkage. In wheel-shaped, cellular ceramic, heat exchangers which are, for example, 36 inches in diameter, 4 1/2 inches thick with a 2 1/2 inches diameter axially disposed hub opening therethrough, the heating during the firing cycle proceeds from the surfaces and especially from the edges formed at the intersection of the upper and lower surfaces with the rim and hub wall surfaces. Internal, unexposed portions of the heat exchanger lag in temperature behind the exposed surfaces and edges, and the differences in time at temperature due to the varying lag behind furnace temperature increases the shrinkage locally at the surfaces and the edges relative to the internal portions of the matrix. The shrinkage of the external portions nearest the hub opening thereby put the internal portions into tension and may result in fracture of integrally formed cellular ceramic or delamination of layered cellular ceramic matrices. Testing has shown that a very high percentage of all delaminations in layered cellular ceramic matrices occurs within a distance of one-half the matrix radius from the central axis.
Increased shrinkage of the rim of the heat exchanger would normally result in compressive stresses on the periphery, but the effects of the compression are not as severe as the effects of a central tension. In addition, accelerated heating around the rim is often retarded by ringing the perimeter with refractory bricks during firing.
Methods of fabricating cellular heat exchangers by layering ceramic green sheets are disclosed in U.S. Pat. Nos. 3,112,184 (Hollenbach) and 3,251,403 (Smith) and are incorporated herein by reference. In general, the method of fabricating a disc-type matrix comprises wrapping alternate layers of flat and corrugated or crimped flexible carrier sheets, coated with a pulverized ceramic material suspended in a binder, circumferentially around a rotating shaft or hub. Smith further suggests that a rigid covering sheet of ceramic material be joined with a foamed ceramic cement on surfaces parallel to the direction of unobstructed cells. In a disc-type heat exchanger with unobstructed cells extending parallel to the central axis, the rigid covering sheets would be covering the periphery or rim of the matrix and the surfaces or hub walls defining the hub or shaft opening. The resultant fabricated matrices are thereafter heated to a sufficiently high temperature to sinter the ceramic material to a unitary structure.
Known ceramic coating materials for the carrier sheets used in fabricating the matrices include low coefficient of thermal expansion materials such as the lithium aluminosilicates, especially glass or crystalline petalite and beta spodumene, or glass-ceramics having the lithium aluminosilicate base and in particular those made according to Example 1 of U.S. Pat. No. 2,920,971 (Stookey) and according to U.S. Pat. No. 3,600,204 (Beall et al.), which are incorporated herein by reference.